The differential diagnosis of an anterior mediastinal mass includes lesions involving the thyroid, parathyroid, thymus, lymphatic, nervous, or vascular structures. The most common tumors include benign and malignant tumors of the thyroid, parathyroid, and thymus, as well as lymphoma and sarcoma. Cystic lesions usually originate in the thymus, a substernal goiter, or are bronchogenic. We present a case of a large parathyroid cyst causing tracheal deviation that masqueraded as a classic substernal thyroid.